Model ST-4 Star Tracker Imaging Camera Operating Manual Table of Contents Instrument Overview ............................................................................. Page 1 Star Tracking Operation ......................................................................... Page 4 Imaging Camera Operation ................................................................... Page 8 Host Computer Software Overview...................................................... Page 10 Using the ST-4 with an IBM PC or Compatible ................................... Page 18 Using the ST-4 with a Macintosh........................................................... Page 28 Observing Suggestions with the ST-4 Imaging Camera...................... Page 37 Problem Solving with the ST-4 .............................................................. Page 38 Appendix A ............................................................................................. Page 39 SANTA BARBARA INSTRUMENT GROUP PO Box 50437 · 1482 East Valley Road, Suite 33 Santa Barbara, CA 93150 Phone: (805) 969-1851 · FAX: (805) 969-4099 Email: sbig@sbig.com · Web: www.sbig.com Model ST-4 Star Tracker / Imaging Camera Operating Instructions INSTRUMENT OVERVIEW The Model ST-4 Star Tracker / Imaging Camera is a multipurpose instrument. It can be used as an automatic star tracker to take long guided exposures of the night sky, or, in conjunction with a personal computer (PC), as a highly sensitive imaging camera. This manual describes the technical concept of the Model ST-4, the interfaces to the telescope and computer, and general operating instructions. Tracking Overview The Model ST-4 uses a charge coupled device (CCD) detector to detect star images. The detector can be understood by examining Figure 1. An array of 192 by 165 detector elements, called pixels, is arranged as shown in the figure. We refer to the horizontal axis as X, and the vertical axis as Y. X Y 165 Pixels position 121,132 the computer will determine its position by noting the increased signal from that pixel. If the star is drifting due to guiding errors, it will appear at a different position in the next exposure, say at pixel position 123,132. The computer then calculates how far the star has drifted from the original exposure, and toggles the telescope drive accordingly. The microcontroller (in the ST-4) can take an exposure (called integration), read out all the pixel values, and calculate the necessary telescope correction in less than a second. The tremendous sensitivity of the CCD enables guide stars as faint as 8th magnitude to be tracked with a 1 second exposure and a 60 mm guide telescope. The calculating power of the ST-4 enables the star's location to be determined to a fraction of a pixel accuracy, enabling very accurate tracking. Imaging Overview 192 Pixels Enlarged View Showing Pixels Figure 1 CCD Configuration In operation the pixels convert photons into electrons, and store them until read out by the ST-4's microcontroller (the ST-4 has memory for two images, one for light frames, and one for dark frames, as explained further below). For example, if a star's image is present at pixel SBIG ST-4/0490 The Microcontroller referred to in the tracking section is built into the ST-4. This microcontroller can communicate to an XT or AT compatible PC, or a Macintosh* , over the ST-4's RS-232 serial link (the serial interface of the ST-4 is also compatible with the RS-422 ports used on the Macintosh). A full image can be transmitted at 19.2K baud within 18 seconds and over a distance of 100 feet. Data transfer rates as high as 57.6K * Macintosh is a registered trademark of Apple Computer, Inc. Page 1 SBIG baud are supported, and work very well over shorter cable runs. A partial image transfer mode is also supported which sends 1/4 as many pixels (one value for each group of four adjacent CCD pixels) in 5 seconds. This mode of operation is very handy for focussing the telescope and finding objects. Finding faint objects is easy using this method; the outline of the Ring Nebula is clearly seen in exposures as short as 10 seconds with an 8" Schmidt Cassegrain Telescope (SCT) operating at f/10. In imaging mode, the microcontroller in the ST-4 is told to take an exposure by the PC. It does so, and stores the resulting data in memory within the instrument. This data is then relayed over the serial link to the host computer. The data is retained in the ST-4 until the next exposure is captured or power is turned off. The host does all of the extensive data manipulation required by the user, such as contrast enhancement, and also can store data on disk for later study. This is a very attractive feature; the computer allows a quick look to be taken at the image, within seconds of the event, and a more detailed look at any later time, such as a rainy night or during the day. The CCD can be exposed for integration times longer than five minutes. The pixels slowly fill-up in the dark due to a phenomena called dark current, and they saturate at about the five minute point when the CCD is cooled to a temperature near -30 °C using the single stage thermoelectric cooler used in the ST-4. The waste heat of the thermoelectric cooler (about two watts) is dissipated into the air by convection around the CCD head. The CCD has a sensitivity comparable to ASA 20,000 film, if such a film speed were available. The CCD has a limited resolution due to the small number of pixels; much greater resolution would be degraded by the limitations of most computer graphics screens. SANTA BARBARA INSTRUMENT GROUP Note: With the CCD running at lower than ambient temperatures, you will wonder why dew and frosting aren't a problem. First of all, the chamber containing the CCD is small, and only a small volume of air surrounds the CCD. The small volume minimizes the total amount of water vapor in the air, which will frost onto the coldest surface inside the head (which is the bottom of the CCD). Although frost may initially form on the top of the CCD, in a matter of minutes it will migrate and be trapped at the back of the CCD. System Interfaces The following equipment is a prerequisite to running the ST-4 CCD Star Tracker / Imaging Camera. 1. A Telescope with pushbutton or joystick slow motions in at least Right Ascension and and hopefully also Declination. A guide telescope, 50mm aperture or larger, or an off axis guider arrangement. 2. For pushbutton type controllers the ST-4 tracker controls the telescope the same way you do; through the RA and Declination slow-motion adjustment switches, the interface to which is shown in Figure 2. Four relays in the ST-4 are used to operate the switches. Most telescope drives have two Declination motor adjustment switches which are normally in the 'open' position. Pushing the button or closing the relay both apply voltage to the motor. Study Figure 2 carefully, along with your pushbutton control, to determine the correct configuration. Most telescopes have one Right Ascension switch which is normally closed; opening this switch slows down the drive. The other Right Ascension switch is normally open; closing this switch speeds up the drive. It is apparent that the relay contacts which are brought out on the cable, 4 groups of three (normally open, normally closed, and common) are all that is necessary to control the telescope. The cable pinouts are described in Appendix A. When the ST-4 tracker is connected to the telescope, the hand controller is not disabled, and SBIG ST-4/0490 Page 2 SBIG still operates normally. If the telescope control is modified for the ST-4, and the ST-4 is unplugged, the drive may not run since the normally closed connection in the ST-4 has now been removed. If this situation is a problem it is best to build up a mating connector to replace the ST-4 box that has the appropriate pins shorted together (usually just two are required to enable the RA drive to work). For joystick type controllers, the four relays in the ST-4 are used to simulate the joystick being pushed to its limits. Two relays are used for each axis or rheostat of the joystick as shown in Figure 3. For the Right Ascension rheostat you would use the +X and -X relays, and for the Declination rheostat you would use the +Y and -Y rheostat. A: Standard Hand Controller Switch c common SANTA BARBARA INSTRUMENT GROUP A relay c A rheostat B C no c nc B nc relay no C A: Standard Joystick Figure 3 Joystick Interface B: Modified Joystick switch nc no normally open normally closed B: Modified Hand Controller Switch c common c nc relay no normally open normally closed nc no Figure 2 Pushbutton Interface SBIG ST-4/0489 Page 3 SBIG STAR TRACKING OPERATION The instrument panel is illustrated in Figure 4. The ST-4 instrument is furnished with a power cable, a cable to the CCD, and a third cable for the telescope's hand controller. There is no power switch; plugging in the power turns the instrument on. The instrument will come-up in FIND AND FOCUS mode, which displays the greatest pixel value found in the readout of the CCD array, and the location of that pixel. The greatest pixel reading will drop as the CCD cools in temperature. After about 2 minutes, the CCD will have cooled to its optimum temperature. Note: The display values range from 0 to 99 corresponding to percentage. Pixel X and Y values of 50 correspond to a pixel centered within the CCD, while a brightness value of 99 corresponds to a completely saturated pixel, i.e., the star is too bright for tracking. 1. 2. SANTA BARBARA INSTRUMENT GROUP Instrument Startup for Tracking Purposes Insert the CCD head into the eyepiece tube such that it seats accurately against the tube. The CCD will not be damaged by light when powered, so it can be easily handled. Begin calibrating the CCD by completely blocking the open end of the telescope to remove all light (the CCD is very sensitive it will saturate in very low ambient light levels). 3. Observe the VALUE reading. This reading should fall to a reading of around 10 and stabilize within two minutes of the instrument being powered-on. Press the INTERRUPT button to halt the FIND AND FOCUS mode (or any other mode for that matter). 4. Push the TAKE DARK FRAME button. The microcontroller will readout the CCD and put the data in the dark frame memory. SBIG TAKE DARK FRAME FIND AND FOCUS CALIBRATE DRIVE TRACK ASTRONOMICAL INSTRUMENTS 89 VALUE 45 X 37 Y STAR TRACKER/IMAGING CAMERA INTERRUPT MENU ADJUST MODE SELECT Figure 4 Instrument Panel 5. Push the FIND AND FOCUS button and uncover the telescope to begin the collection of light by the ST-4. The instrument will automatically begin taking frames of CCD data, subtract the dark frame stored in memory, and display the maximum value. Direct the telescope to a star and adjust the telescope's position to approximately center the star image on the CCD by noting the reading on the X and Y displays. 6. If the star is too bright (VALUE reading 99) then either the exposure must be reduced, or a fainter star chosen for tracking. In order to correct this condition, press the INTERRUPT button to stop the collection of data, and Page 4 SBIG ST-4/0490 SBIG return control of the ST-4 instrument to the keyboard. Push the MENU button. The brightness display will then read "EA" (exposure adjust) and a "1" will appear on the X pixel display (indicating a default exposure time of one second). Repeatedly pushing the ADJUST key will scroll through a list of exposure times, from 0.1 second, to 20 seconds. Adjusting the exposure time to shorter times will reduce the star brightness, while choosing a longer exposure time will increase star brightness. When the desired exposure time has been chosen, press the MENU button again (the ST-4 will display "CA" (Calibration Adjust), and then press it several more times until you see a bA displayed in the value box. bA stands for brightness adjust; two modes are available, A for average, or F for faint. F increases sensitivity by 9 times. You should set this parameter to A for initial familiarization. Press the MENU button again until you see the boost (b) parameter displayed. This is a boost factor, where greater values mean greater gain. Initially set this value to 1. Press the MENU button again to return to the normal operating mode (after being interrupted "HELLO" appears on the display). Note: You must take a new dark frame if you change the exposure time, the brightness adjust, or the boost factor. 7. When the brightness level has been adjusted to an acceptable level, focus the telescope by turning the focus knob and observing the VALUE display. At best focus, this number is maximum. Be careful to take your hand off the telescope between adjustments or the telescope vibration will smear the star image over multiple pixels within the CCD, reducing the brightness. Atmospheric turbulence will also tend to smear the image, so it may be helpful t ...